Personally I think this borders on criminal stupidity. Good instructors can effectively demonstrate all of the required skills while maintaining a good reserve of safety margins to allow for the unexpected.

BPF
Canadian glider License with Instructor and Aerobatic Instructor ratings.

Sorry for the thread drift but I had to comment.

Back to the thread topic......my 2 cents

I found the thread rather entertaining and as usually happens it forced me to evaluate and consolidate what I knew.

However flying is an inherently practical exercise in hands and feet an coordination. for light aircraft I say again for light aircraft I firmly believe that there inherent low inertia means that all of the physical effects discussed at length in this thread exists to such a small extent that they are effectively theoretical not actual.

The one effect that is very real and regularly kills is the visual illusion.

SSD summarized it nicely on page 3

You just can't convince some people BPF. Earlier I made a post re a 777 operating in a 200 knot jetstream and statedto which Heston repliedYour quoting of SSD has it absolutely correctYouris the killer.

For Heston, I never flew a jet, a humble ex helicopter pilot me, and the downwind turn is a killer in helicopters as well, though for different reasons to a fixed wing. As I related earlier we took off at times in 60 knot winds with a climb speed of 75, meaning a GS of 15 into wind and 135 downwind. Most of our copilots only had a bare CPL on joining, and the demonstration of a take off in those conditions, with a bank angle of 60° applied at climb speed to downwind gave an extremely impressive demonstration of the need to fly instruments and ignore the visual. The ASI remained nailed on climb speed, and the ball remained centred throughout.

It matters not if you are in a 777 in the jetstream, or a bug smasher at sea level. The physics are exactly the same.

Sorry Megan, Heston is right.

If you change the ground speed, you have accelerated. You can accelerate without changing your airspeed. You do it every time you make a turn - the aircraft is accelerating towards the centre of the turn. Acceleration is rate of change of velocity - and velocity (unlike speed) is a vector - that is it has a direction as well as a size. So if we change direction, we accelerate.

That's Physics for you.

But you're quite right about the rest - make your turn whilst ignoring the visual illusions and so on, and the ASI will stay nailed.



Paul

Isn't acceleration an increase in speed relative to the medium that the body is suspended by? Rather than an acceleration relative to something else?
You are accelerating towards the centre of the turn but the centre of the turn is moving, so what ever else you use as a reference is irellevent.
You cannot use two points of reference at the same time.

And using the wrong one is the fatal bit... Use the ASI, not the ground.

Acceleration = the rate of change of velocity per unit of time

Sorry, thats bollox for you.

The problem with that is that you have NOT changed your airspeed. The airspeed is the medium you require to pay attention to. That is the medium that you are flying in. The physics you quote, is only applicable on the ground, i.e. in the same medium.

Take a car. If the car turns, there is an acceleration toward the new direction, we can call the forward acceleration of the car a "linear acceleration", which passengers in the car might experience as a force pushing them back into their seats. When changing direction, we might call this "non-linear acceleration", which passengers might experience as a sideways force. It is FORCE, not acceleration.

The fact that your aeroplane may increase groundspeed, is, IMO, and that of Newton I think, not acceleration.

Thats physics for you...

My instructor teaches speed by attitude with ASI as a delayed and sometimes erratic backup. The ground doesn't even come into it and I don't notice it in the circuit. Horizon and aiming point are what matters.

Might I hazard a guess, Maxred, that you're not a physicist?

That force that car passengers experience? Caused by acceleration - whether it's along the line of travel or sideways. (Newton's second law - F=ma). That 2g you feel in a 60 degree banked turn - it's the aeroplane pushing your backside so you accelerate along with it.

I didn't claim for a moment that your airspeed would change. Other things being equal, in a turn, it won't.

But if your groundspeed is changing, you will be accelerating. Even if the airspeed stays the same. And no matter what speed the wind is.

And the idea that the physics is only applicable on the ground will, I suspect, be a surprise to most physicists! They do rather cling to the idea of the physics holding true no matter what frame of reference you use.

Paul

acceleration is by definition the rate of change of velocity, as measured in an inertial frame of reference (ie a non-accelerating frame). The issue people seem to be having is what is 'velocity', which I think is the root of the differences of opinion.

The conventional frame for measuring Velocity is the ground (which is very close to inertial after adjusting for a 1 g offset from gravity). So, if an aircraft has changed its ground velocity, it has by definition accelerated.

If the air mass is moving at a constant velocity (with respect to the ground), measuring velocity relative to the air mass will give exactly the same acceleration as measuring vs the ground.

If you do the maths for two cases of a level constant rate turn
1 - flying North at 60 knots and turning to South at 60 knots
2 - flying North at 0 knots (into a 60 knot headwind) and turning to South at 180 knots

You will find the answers are all identical, except the track over the ground is displaced by a constant distance of 60 nautical miles every hour in the second case.

In both cases the air SPEED (not air VELOCITY) will remain constant. The Relative Air Velocity (measured relative to the wing, which for normal rates of turn is effectively an inertial frame) will remain constant. It is this relative Velocity that defines the performance of an aircraft wing.

If the air mass is not moving at a constant speed, the aircraft will require accelerations to maintain the relative air velocity, hence the importance of the aircraft's inertia when exposed to gusts and shears.

Thanks MM. There you go Paul.........apologies, I am not a physicist. Got a well earned B in my A level though.....I am teasing a bit, obviously, and Paul, I admire your argument based on pure physics. I am sure everyone is now totally bored and baffled with the physics, but the scenario is a serious one. If anything this thread could achieve, is that it gets folks thinking about it......

You may want to have a butchers every now and then.

You wake up, not sure where you are. You see that you are in a small, windowless room. Apart from your chair, the room is empty.

On the floor, you find a small model aeroplane with a battery-powered motor driving counter-rotating propellers. A simple instruction sheet tells you that when the motor is switched on and the 'plane is hand-launched, the controls are fixed so that it will fly perfect circles until a timer turns the motor off after 30 seconds.

You launch the model and watch, pleased and impressed, as it flies circles around you.

After the flight has ended, you notice that there is actually a window blind in the wall behind you. You open the blind and are amazed to find that you are in fact inside a carriage being pulled by a train along a perfectly straight, smooth track at 60mph.

You consider the fact that the model aircraft was flying in a parcel of air which was moving across the ground at 60mph. To an observer not on the train, it was, in effect, flying in a 60mph wind. This is a speed which is about five times greater than its own flight speed, yet the model did not seem to be affected at all! It did not, for example, exhibit any signs of 'stalling' or losing height when it was turning 'downwind'.

You come to the conclusion that the model was simply unaware of its location and speed relative to the tracks, as were you until you looked out of the window.

Just to keep the record straight. I did not actually state that.........

That's fine, but I suggest this may not be the best policy if the weather closes and you have to make a low-level circuit on a windy day :ooh:

Well, if you really want your head to hurt.....

As I'm sitting typing this, I am in fact accelerating at 1g upwards, pushed by the force from my chair. Relative to me, the inertial frame of reference is falling towards the centre of the earth at 1g, as I would be (following a straight line in space time), were it not for the earth getting in the way.

But then we'd be having a physics conversation, not a flying one. And we'd be using Einstein's physics, rather than Newton's. The latter works quite well for our purposes, and the former might make the PPL exams a bit tricky.

Paul

The thinking power being brought to bear on the acceleration topic brings to mind a question I've had in my mind for years. It's really an astronaut question, and I came close to being able to ask Chris Hadfield at his presentation, but he sure is popular with the crowd! Anyway:

The space shuttle, which can maneuver in orbit, is orbiting the earth, and the astronauts inside are floating around, also orbiting the earth (weightless, but held in orbit by G? - no that's not the question). If the space shuttle pilot maneuvers the space shuttle in its orbit (I think they roll it in and out of the sun for heating/cooling?), would he have to tell all the astronauts to hang on? Then they would experience acceleration as they change direction with the shuttle? Otherwise, it would maneuver around them - and maybe hit them, as they remained in the original orbit...

When my daughter was quite young, she could be entertained until I felt airsick, with my placing Mr. Bear on the glareshield, and then bunting over in near zero G so he seemed to jump into her lap all on his own. Was I placing Mr. Bear in a very crude orbit for a few seconds, surrounded by a C 150?

Yes, and yes.

The shuttle direction change is an acceleration which requires force. The astronauts must have some force applied for them to stay with the shuttle's new direction.

Zero G occurs when you accelerate downwards at 9.8ms^2. Google the vomit comet for another example.

Think sideways force, and constant acceleration.......

Maybe I should have spelled it out that I was referring to turns in the circuit in VMC and maintaining healthy airspeed.

PaulisHome, hate to tell you but Heston, as are you with

are incorrect. You are not accelerating. There are two frames of reference, in both of which Newtonian physics apply. One frame is the earth, the other is the airmass, and for an aircraft in flight the relevant frame is the airmass. The only acceleration an aircraft experiences when making a turn, while maintaining a constant airspeed, is that of "g" in its vertical axis, being 2 "g" for a balanced 60° banked turn. Have a read up on Galilean invariance. eckhard in his post re the model in a train exemplifies exactly the Galilean invariance.

People here seem to be confusing speed and velocity.

Acceleration is the name we give to any process where the velocity changes. Since velocity is a speed and a direction, there are only two ways for you to accelerate: change your speed or change your direction—or change both.

An aircraft in a turn is accelerating.

Hi Megan

You seem confused. On the one hand you say "You are not accelerating". On the other, you say "The only acceleration an aircraft experiences ... is that of "g" in its vertical axis, being 2 "g" for a balanced 60° banked turn" Which is it? No acceleration or 2g?

Let me help. It's the second. That 2g isn't pointed upwards relative to the earth (or we'd be accelerating in a loop), it's pointed at 60 deg from the vertical. That resolves into 1g vertical to the earth which counteracts gravity, and about 0.87g pointing towards the centre of the turn. It's the second one which accelerates us around the turn. (and just FYI, the radius, velocity and acceleration are described by the formula a=v^2/r).

And Fujii is entirely correct in #142

I don't think you've interpreted Gaililean equivalence properly. What it means is that the laws of motion apply no matter what frame of reference you use. There is no correct frame of reference. Using the airmass frame of reference makes the sums easier, sure, but it's quite possible to use the ground, or any other, and it won't change what actually happens. mm_flynn demonstrated that in #130

Paul

Paul, I thought the discussion was in reference to velocity ie in the fore/aft axis. So the question is, what reading would an accelerometer placed in the fore/aft axis read while maintaining a constant airspeed and turning from into wind to downwind? + something, - something, zero? Forget the vertical and lateral.

And I should have noticed that you are under instruction, HR, I'm sorry if my comment seemed flippant. I'm enjoying this learned discussion but I still think myself very fortunate to have had a brilliant ex-RAF instructor who demonstrated the misleading visual cues which have entrapped so many pilots when turning downwind at low level, especially with a misty horizon or none at all. It also helped that the nearest controlled airspace was 30 miles to the west, and Blue Two five miles above. Happy days ... I wish you just as many in your flying career although I understand ATC tends to frown on low-level instructional circuits over the parked Airbuses, in strong winds or otherwise.

It would read zero. All the acceleration is normal to the aircraft as you imply. Airspeed would stay the same, groundspeed would vary (assuming there was a wind).

Still true to say that if the ground speed varies, the aircraft is accelerating. That isn't the same as saying that its airspeed is changing. It isn't though necessarily true to say that if the groundspeed stays the same, the aircraft isn't accelerating.

(I can think of only one, rather extreme, example where the first of those isn't quite true, and that's if we head north/south, in which case the speed of the ground moving under us due to the rotation of the earth changes).

But we're having a physics discussion at this point, not a flying one, and we need to use our terms carefully.

;)

Paul

And THAT is the only point that needs making in this absolutely ridiculous string of threads born from the Perth Mallard crash. The stuff that is passing over the wing and keeping you up in the air is NOT changing and all the rest of the chatter that surrounds this one fact is just chaff. The downwind turn is a myth generally spouted by inexperienced instructors, believed as true by their even more inexperienced students and so continues to evolve as 'truth'. It's all complete and utter bollox.

Keep the speed right (IAS), keep the ball in the middle and don't pull further than the light buffet.

From the wings. I'm using the proper definition of acceleration here (not just confined to the fore/aft axis). So if the ground speed is varying then either the airspeed is varying, or the direction of the aircraft is varying and there's some wind. In either case there is acceleration.

P

The discussion so far has mostly focussed on still air, or a parcel of air moving at a consistent speed. The advice that in these conditions turning up or downwind makes no difference would tie up with my limited experience - I haven't found the need to "dolphin" around a thermal!

However, what about in gusty conditions? If I turn into wind, as I fly through a gust head-on I would expect the ASI to flicker up (although by the time it registers I'm probably out the other side!). However, if I turn downwind, would I not "overtake" a gust, which should register in a brief drop in airspeed? I would also have thought that it would take longer to pass through the gust, as it's travelling in the same direction. So in these conditions, "average" airspeed would be lower turning downwind that up?

PaulisHome. Re post #146, don't accelerometers read one when at rest or is it different for non aviation accelerometers?

Yes, if you are referring to measuring the "g" in the vertical axis, but when placing an accelerometer in the lateral or longitudinal axis they would read zero when the aircraft is at rest. For example, in the longitudinal axis it would record a value as the aircraft accelerates for take off, or de-accelerates upon landing, and any other the time the aircraft increases/decreases speed. One placed in the lateral axis would record a value when making a turn during taxi, or if slipping or skidding in a turn during flight ie the turn was not balanced. Police measure a "g" following accidents to record the available braking performance of the road surface ie the de-acceleration available.

Here's a real example that you can try at home. The free 'geemeter' iPhone app displays the readings from one of the three accelerometers in the iPhone.

The three pictures below show the display with the iPhone in three different orientations:

1. Held vertically in front of you +1.0 G
2. Held upside down -1.0 G
3. Flat on a table 0.0 G

An alternative is to use the free iSeismometer app and look at the output of each accelerometer as you rotate the iPhone about all three axes.

To quote Elmer Shakespeare. "Methinks y'all doth think too much". Nerd Forum


PS. What's de-acceleration ?

Good question. I think it depends on how you model the gusts. If we model the gust as instantaneous say +/- 5 kts in the horizontal layer, say for 100m (don't ask how for a moment), then despite the airspeed changing the aircraft is still moving at the same speed as the overall airmass frame of reference (that inertia thing). So it would take the same time to move through the 100m no matter which way the gust was going.

If you think about a thermal - the model for that is a vortex smoke ring - so depending on where you hit it you can find air going up, down, in or out. But the thermal as a whole will be moving at the same speed as the airmass, so if you go through it symmetrically and relatively quickly, you'll spend the same time with air gusting towards the nose as away from the nose.

P

No problem Geri. I should also point out that my instructor is reluctant to teach when the horizon isn't clear until the student has the 'feel' for the right level, climbing and descending attitudes. I feel happy learning with the ASI as backup only.
Maybe this is why I don't understand the influence of the ground on the perception of airspeed.

deacceleration
English
Etymology

de- +‎ acceleration
Noun

deacceleration (plural deaccelerations)

The act of deaccelerating; retardation

decelerate. Look it up.

I'm familiar with Deceleration. Just hadn't encountered Megan's term before. De-acceleration.
Thought it might mean something different. Like returning to a constant speed?
That's what happens when you try to apply logic to English grammar.:)

As this has turned into a physics discussion of two frames of reference, I've been having a think, dangerous thing to do!
A ship sailing across an aqueduct displaces its 100tons weight in water, no additional weight added to the aqueduct. The ship sinks 2inches in the middle of the aqueduct and is resting on the bottom, does the aqueduct collapse under the extra 100tons weight? Or does it still weigh the same? Or does it weigh more by the difference in displaced water?

It's no longer a distributed load (applied by depth of water x density x g) but a point one, so a different loading case.

Now, what about banging on the side of a 15cwt truck to transport a ton of budgies?

Where does the displaced water go? If 100tons of water is sloshed over the sides, no change of total weight. If 100tons of water is displaced by raising the level of the aqueduct but all the water is still in situ; net gain of 100tons I would have thought.

If 100tons of water is displaced longitudinally (along the length of the aqueduct) then maybe there would be no weight gain in the section under discussion. Is the length infinite? Does it have to be? My brain hurts.